Support extended persistence

[wreise]

Description

Some use cases involve producing diagrams, with points under the diagonal - for example, with superlevel-set filtrations. Do we want to extend the API to offer this?

Currently, the users can modify the metric by hand, to invert the order that the edges appear in. But, the features (Betti Curves etc.) require the persistence diagrams to satisfy birth<=death.

One idea to solve these problems is to extend the homology and diagrams API to extended persistence (using negative dimensions to indicate the extended part).